MT#61856 docs: re-design general tree

Make entry tree simpler and give
links to commonly used things like
NG control protocol.

This simplifies usage of documentation.

Change-Id: I1283bf8a88c7c6e4ff7ab9451610bc6e224c0119

docs/index.rst

@@ -8,10 +8,31 @@ Welcome to the Rtpengine Project Documentation
 .. toctree::
    :maxdepth: 4
 
-   overview
-   rtpengine
-   rtpengine-recording
+   Overview <overview>
+
+==============================================
+
+.. toctree::
+   :maxdepth: 1
+
+   usage
+   Option flags (NG control protocol) <ng_control_protocol>
+
+==============================================
+
+.. toctree::
+   :maxdepth: 1
+
+   Daemon manual page: start and configuration options <rtpengine>
+   Recording daemon manual page: start and configuration options <rtpengine-recording>
+
+==============================================
+
+.. toctree::
+   :maxdepth: 2
+
    architecture
    tests
    troubleshooting
+   transcoding
    glossary